Fix broken dependency settings.
Rework HTTP API.

- Create a new User controller and move most business logic there.
  - Add new routes for `/users` that point to this controller.
- Controller action Application.index just redirects to the User
  controller.
- Add tests for User controller.
- Reduce test scope for Application & User controller.
  - Move 404 test to IntegrationSpec where it belongs.
- User creation no longer results in a (pointless) redirect.